High performance heat pump unit

ABSTRACT

A heat pump unit ( 1 ) comprises at least one main circuit ( 2 ) adapted to perform a main heat pump cycle with a respective operating fluid, which comprises: a main condenser (S 4 ) adapted to perform the condensation of the operating fluid of the main heat pump cycle and intended to be connected to an external circuit of a first thermal user plant ( 10 ) in a heating operating mode of said heat pump unit ( 1 ); a first heat exchanger (S 3 ), connected downstream of the main condenser (S 4 ) and upstream of expansion means (L 2 ) of said the main circuit ( 2 ), adapted to perform an undercooling of the operating fluid of the main heat pump cycle after the condensation of the same in the main condenser (S 4 ), and a main evaporator (S 8 ) adapted to perform the evaporation of the operating fluid of the main heat pump cycle and intended to be connected to an external circuit of a heat sink ( 20 ) in a heating operating mode of said heat pump unit ( 1 ). The first heat exchanger (S 3 ) is selectively connectable to the external circuit of the first thermal user plant ( 10 ) so as to be in series with the main condenser (S 4 ) in said external circuit.

FIELD OF THE INVENTION

The present invention relates to the field of heat pumps. In particular, the invention relates to a heat pump unit adapted to be used for heating/cooling environments and for producing sanitary hot water with high performance in terms of energy efficiency and use flexibility.

PRIOR ART

Heat pumps are an increasingly widespread technical solution for meeting the requirements of heating/cooling environments and/or fluids. The reasons for such success are mainly to be ascribed to the high energy efficiencies, to the possibility of using a single device for both heating and cooling (so-called “reversible” heat pumps), to the flexibility in managing thermal users with different requirements and to the possibility, in case of use for heating, of considerably reducing the use of fossil fuels and thus the outlet of greenhouse gases harmful to the environment.

In order to make the use of heat pumps increasingly competitive, the focus of designers and manufacturers is on a constant improvement of the performance thereof, both in terms of energy efficiency and in terms of use flexibility (possibility of use for both heating and cooling, possibility of meeting multiple different requirements of multiple thermal users, even concurrently, in terms of heating/cooling power and/or operating temperatures, capability of operating at partial loads without energy efficiency degradation, etc.). The optimization need is especially felt for heat pump units having a high heating/cooling power (for example >100 kW), typically intended to be used in large buildings with centralized thermal users, such as blocks of flats, hotels, hospitals, barracks, sports centers, swimming pools, etc.

In the case of gas compression heat pumps intended for heating, a known method for improving the COP (Coefficient of Performance) consists in performing an undercooling of the operating fluid after the condensation thereof and in using the undercooling heat power thus obtained for preheating the heat carrier fluid coming from a heat sink before sending it to the evaporator for determining the evaporation of the operating fluid.

Documents DE 3311505 A1 and WO 2011/045752 A1 describe the use of the above solution in particular in so-called “high temperature” gas compression heat pumps. Such heat pumps allow condensation temperatures of 80-85° C. to be achieved—required for the operation of conventional high temperature heating plants which typically require a delivery temperature of the heat carrier fluid of at least 80° C.—even when a heat sink is provided the average temperature of which does not exceed 7-10° C., as it normally happens with groundwater. Two stage heat pumps are typically needed in order to operate with so large differences, which however usually have relatively low COP.

In two stage heat pumps described in the above documents there is provided an additional heat exchanger connected downstream of the condenser and upstream of the expansion means in the circuit of each stage. The additional heat exchangers are further connected to a delivery line of a heat carrier fluid of a heat sink, upstream of the evaporator of the lower temperature stage. It is therefore possible to preheat the heat carrier fluid coming from the heat sink before sending it to the evaporator of the lower temperature heat pump cycle through the heat power resulting from the undercooling of the operating fluids that perform the higher and lower temperature heat pump cycles. Thanks to such configuration it is possible to obtain COP equal to or higher than 3 even in two stage heat pumps.

SUMMARY OF THE INVENTION

The technical problem at the basis of the present invention consists in providing a heat pump having improved performance compared to the heat pumps having the same power and type of the prior art. In particular, a heat pump is desired which is capable of ensuring a high energy efficiency, with COP in case of heating or EER (Energy Efficiency Ratio) in case of cooling equal to or higher than 3, in a wide range of operating conditions, also in the presence of thermal users with different requirements in terms of heating/cooling power and/or operating temperatures required.

The Applicants have perceived the possibility of solving such technical problem using the heat power resulting from an undercooling subsequent to the condensation of the operating fluid in a heat pump cycle in an alternative and more effective manner compared to the solution presented in the prior art described above.

The invention therefore relates to a heat pump unit comprising at least one main circuit adapted to perform a main heat pump cycle with a respective operating fluid, said at least one main circuit comprising:

-   -   a main condenser adapted to perform the condensation of the         operating fluid of said main heat pump cycle and intended to be         connected to an external circuit of a first thermal user plant         in a heating operating mode of said heat pump unit;     -   a first heat exchanger, connected downstream of said main         condenser and upstream of expansion means of said at least one         main circuit, adapted to perform an undercooling of the         operating fluid of said main heat pump cycle after the         condensation of the same in said main condenser, and     -   a main evaporator adapted to perform the evaporation of the         operating fluid of said main heat pump cycle and intended to be         connected to an external circuit of a heat sink in a heating         operating mode of said heat pump unit,

characterized in that said first heat exchanger is selectively connectable to the external circuit of said first thermal user plant so as to be in series with said main condenser in said external circuit.

Within the scope of the present description and in the following claims

-   -   the expression “heat pump cycle” is understood to indicate a         generic inverted thermodynamic cycle, i.e. a thermodynamic cycle         adapted to transfer heat power from a means or system at lower         temperature to a means or system at higher temperature, or in         order to increase or keep the temperature of the means or system         at higher temperature high (heating operation), or in order to         decrease or keep the temperature of the means or system at lower         temperature low (cooling operation), and     -   the expression “heat sink” is understood to indicate a means or         system capable of yielding or absorbing heat power without         considerable variations of the average temperature thereof.

The heat pump unit of the invention advantageously allows the use of the heat power resulting from an undercooling of the operating fluid after the condensation thereof for preheating the heat carrier fluid of a thermal user plant before it reaches the main condenser.

The Applicants have also found that such use of the undercooling heat power, substantially different from the use thereof made in the prior art heat pump units (preheating of the heat carrier fluid of the heat sink) may have a considerable positive effect on the COP of the heat pump unit, especially in specific operating conditions.

Such positive effect is related to the undercooling heat power fraction actually usable compared to the theoretically available one, which is determined by the minimum temperature achievable with undercooling, i.e. the evaporation temperature of the operating fluid of the main heat pump cycle.

Since the undercooling heat power usable is greater as the back temperature of the heat carrier fluid to be heated is lower, the heat pump unit of the invention is particularly advantageous in all those operating conditions wherein a drop of such temperature alone or globally of the temperature level in the thermal user plant is acceptable with the same heat power transferred thereto. Such situation occurs, for example, in high temperature heating plants in autumn and spring.

The heat pump unit of the present invention therefore finds a particularly advantageous use in combination with high temperature heating plants which provide for the possibility of changing the backflow temperature of the heat carrier fluid in order to optimize the operation of the heating plant.

Another use situation of interest is in combination with high temperature heating plants which envision high temperature differences between delivery and backflow of the heat carrier fluid.

It has been determined that with a suitable selection of the operating fluid and of the operating parameters, in the above operating conditions and/or application cases, it is advantageously possible to obtain an increase in the COP up to 20% compared to the values obtainable in conventional heat pumps of the same type and power.

The heat pump unit of the invention can therefore ensure high energy efficiency in a wider range of operating conditions compared to conventional heat pumps of the same type and power.

Moreover, the technical features of the heat pump unit of the invention by which it is possible to obtain the advantageous results described above are compatible and easily integrated with other technical solutions aimed to use the undercooling heat power of the operating fluid, such as for example the preheating of the heat carrier fluid of the heat sink carried out in the prior art devices.

In a preferred embodiment of the heat pump unit of the invention, said main circuit comprises a first sub-circuit adapted to perform a higher temperature main heat pump cycle with a respective operating fluid and a second sub-circuit adapted to perform a lower temperature main heat pump cycle with a respective operating fluid, wherein said first and second sub-circuits are in cascading heat exchange relationship with each other to perform globally a two-stage main heat pump cycle, and wherein said main condenser and said first heat exchanger are connected in said first sub-circuit and said main evaporator is connected in said second sub-circuit.

Such configuration of the main circuit allows a two-stage main heat pump cycle to be performed, and thus the operation with thermal gradients significantly higher than those obtainable through a single-stage heat pump cycle. Thanks to this it is advantageously possible to use the heat pump unit of the invention with thermal user plants operating at a high temperature (for example radiator heating plants which normally require delivery temperatures around 80° C.) also when a heat sink is available which consists of water or environmental fluids at low temperature (for example groundwater or running water on the surface or in depth, seawater or lake water, waterworks water, wastewaters, etc., with average temperatures typically not lower than about 7° C.).

In another preferred embodiment, said second sub-circuit comprises a second heat exchanger connected downstream of a condenser and upstream of expansion means of said second sub-circuit, said second heat exchanger being adapted to perform an undercooling of the operating fluid of said lower temperature main heat pump cycle after the condensation thereof and being selectively connectable to the external circuit of said heat sink so as to perform a preheating of a heat carrier fluid coming from said heat sink by means of heat power released during said undercooling by the operating fluid of said lower temperature main heat pump cycle.

The recovery of heat power resulting from the undercooling of the operating fluid of the lower temperature main heat pump cycle for preheating the heat carrier fluid of the heat sink when the heat pump unit of the invention is active in heating implies a further improvement of the COP.

Preferably, said second heat exchanger is further selectively connectable to the external circuit of a second thermal user plant.

In this way, the heat power resulting from the undercooling of the operating fluid of the lower temperature main heat pump cycle may be used for serving a further medium/low temperature thermal user, for example a heating plant with floor or ceiling radiating panels, fan coils, etc. The possibilities of use and the overall energy efficiency of the heat pump unit of the invention therefore are advantageously increased.

Preferably, moreover, said first sub-circuit comprises a third heat exchanger connected downstream of said first heat exchanger and upstream of expansion means of said first sub-circuit, said third heat exchanger being adapted to perform an undercooling of the operating fluid of said higher temperature main heat pump cycle after the condensation thereof and being selectively connectable to the external circuit of said heat sink so as to perform, preferably independently of said second heat exchanger, a preheating of a heat carrier fluid coming from said heat sink by means of heat power released during said undercooling by the operating fluid of said higher temperature main heat pump cycle.

Preferably, said third heat exchanger is further selectively connectable to the external circuit of said second thermal user plant.

These embodiments replicate, in the first sub-circuit for performing the higher temperature main heat pump cycle, what described above with reference to the second sub-circuit for performing the lower temperature main heat pump cycle, advantageously allowing the increase of the heat power available for preheating the heat carrier fluid of the heat sink or a medium/low temperature thermal user to be served.

In a preferred embodiment, said first sub-circuit comprises a fourth heat exchanger connected downstream of said main condenser and upstream of said third heat exchanger, said fourth heat exchanger being adapted to perform an undercooling of the operating fluid of said higher temperature main heat pump cycle after the condensation of the same, and a secondary circuit adapted to perform a secondary heat pump cycle with a respective operating fluid, said secondary circuit comprising:

-   -   a secondary evaporator adapted to perform at least the         evaporation of the operating fluid of said secondary heat pump         cycle and in heat exchange relationship with said fourth heat         exchanger to transfer heat power released by the operating fluid         of said main heat pump cycle during said undercooling to the         operating fluid of said secondary heat pump cycle;     -   a secondary condenser adapted to perform the condensation of the         operating fluid of said secondary heat pump cycle and intended         to be connected to the external circuit of said first thermal         user plant or to an external circuit of a third thermal user         plant, different from said first and second thermal user plants.

Thanks to the arrangement of a fourth heat exchanger and to the performance of a secondary heat pump cycle with the features mentioned it is advantageously possible to bring at least one fraction of the heat power released during the undercooling of the operating fluid of the main heat pump cycle (i.e. of the higher temperature main heat pump cycle in the case of two-stage main heat pump cycle) to a higher temperature, in particular substantially equal to the temperature at which the condensation heat power is released in the main condenser. The useful power that the heat pump unit can provide is therefore increased, since a fraction of the undercooling heat power may be used, in addition to the condensation heat power released in the main condenser, for serving the first thermal user plant, or another thermal user plant operating with similar temperatures, also in operating conditions wherein such plants require the maximum temperature levels provided for their operation.

It has been found that, unlike what happens for example in the case of the cascading coupling of two heat pump cycles according to the prior art, in this case the increase of the above useful heat power leads to an improvement of the overall COP. This is essentially related to the fact that such increase in the useful heat power may be achieved with a minimum additional use of energy, in particular electrical energy for compressing the operating fluid in the secondary heat pump cycle.

In fact, it should be noted that the undercooling of the operating fluid of the main heat pump cycle takes place, due to its nature, with a temperature variation. The heat power released during the undercooling of the operating fluid of the main heat pump cycle therefore allows not only the evaporation but also a strong overheating of the operating fluid of the secondary heat pump cycle, to be obtained.

The overheating, which is stronger as the thermal gradient of the operating fluid of the main heat pump cycle is wider during the undercooling, has two important effects that contribute to a considerable reduction of the electrical compression power in the secondary heat pump cycle.

Firstly, there occurs an increase in the enthalpic jump undergone by the operating fluid of the secondary heat pump cycle in the heat exchange with the operating fluid of the main heat pump cycle in the undercooling step. Having established the heat power to transfer between the two fluids, such enthalpic jump allows a corresponding reduction in the mass flow rate of the secondary operating fluid so that less compression work is required.

Secondly, the overheating distances the operating fluid of the secondary heat pump cycle from the saturated steam conditions and therefore allows the use of compressors with higher isentropic yields, without the risk of intersecting the higher limit curve during the compression.

Both aspects mentioned contribute to a reduction in the electrical power used for the compression of the operating fluid of the secondary heat pump cycle and thus, according to what explained above, to the increase in the overall COP of the heat pump unit of the invention.

In a preferred embodiment of the heat pump unit of the invention, both said main condenser and said secondary condenser are intended to be connected to the external circuit of said first thermal user plant and are connected to one another so as to be in series in said external circuit of said first thermal user plant.

This embodiment advantageously allows the use of both the condensation heat power and of at least one fraction of the undercooling heat power of the main operating fluid for the same thermal user plant.

Advantageously, moreover, with this embodiment it is possible to obtain a further improvement in the overall COP, since it is possible to perform the heating of the heat carrier fluid of the user plant in two steps that occur in a sequence in the secondary condenser and in the main condenser. Since in this case the secondary condenser must only contribute to a part of the heating, the heat power to transfer to the thermal user plant being equal, it is possible to decrease the condensation temperature in the secondary heat pump cycle, obtaining a simultaneous decrease of the compression work required in such cycle and in the practice, an increase in the overall COP of the heat pump unit.

A situation of interest for the use of this embodiment therefore is in combination with high temperature heating plants which envision high temperature differences between delivery and backflow of the respective heat carrier fluid.

Preferably, moreover, the heat pump unit according to the invention comprises switching means, adapted to allow an exchange of connections of the external circuits of at least said first thermal user plant and said heat sink respectively with at least said main condenser and with said main evaporator.

This allows a reversible heat pump unit to be obtained, capable of operating both for heating and for cooling. Advantageously, the choice to perform the cycle reversal by exchanging the external circuits of the thermal user(s) and of the heat sink, respectively, releases the switching between the two operating modes from the specific configuration of the heat pump unit (main circuit with one or two stages, number of heat exchangers connected to a same delivery line of the thermal user plant(s), etc.).

Preferably, the operating fluid of said main heat pump cycle, or the operating fluids of said higher temperature main heat pump cycle and said lower temperature main heat pump cycle respectively, and the operating fluid of said secondary heat pump cycle are selected from the group consisting of: (E)-2-butene, (Z)-2-butene, 1-butylene, dimethyl ketone, methylacetylene, methyl alcohol, methylpentane, methylpropene, n-hexane, R1270, R290, R600, R600a, R601, R601a, RE-170, tetramethylmethane or RC-270.

The above cooling fluids are characterized by limit curves in diagram h-p (specific enthalpy-pressure) strongly inclined towards the increasing enthalpies, with increasing inclination as pressure increases. This advantageously allows even strong undercooling to be performed which, as already explained, allows all the advantageous effects on the overall COP that can be obtained by the above embodiments to be enhanced.

The invention also relates to a system for heating/cooling environments and/or for producing sanitary hot water comprising a heat pump unit having the features described above.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the present invention will appear more clearly from the following description of some preferred embodiments thereof, made by way of a non-limiting example with reference to the annexed drawings, wherein:

FIG. 1 shows a circuit diagram of a first preferred embodiment of the heat pump unit of the invention;

FIG. 2 shows a circuit diagram of a second preferred embodiment of the heat pump unit of the invention;

FIG. 3 shows a circuit diagram of a third preferred embodiment of the heat pump unit of the invention;

FIG. 4 shows a circuit diagram of a fourth preferred embodiment of the heat pump unit of the invention;

FIG. 5 shows a circuit diagram of a fifth preferred embodiment of the heat pump unit of the invention;

FIGS. 6A and 6B show circuit diagrams of two operating configurations of a sixth preferred embodiment of the heat pump unit of the invention;

FIGS. 7A and 7B show circuit diagrams of two operating configurations of a seventh preferred embodiment of the heat pump unit of the invention;

FIGS. 8A and 8B show circuit diagrams of two operating configurations of an eighth preferred embodiment of the heat pump unit of the invention;

FIGS. 9A and 9B show circuit diagrams of two operating configurations of an ninth preferred embodiment of the heat pump unit of the invention, and

FIG. 10 shows a circuit diagram of a tenth preferred embodiment of the heat pump unit of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the figures, a heat pump unit according to the invention is globally indicated with reference numeral 1.

In such figures, the heat pump unit 1 is shown as a part of a system 100 for heating/cooling environments and/or for producing sanitary hot water, comprising at least one external circuit of a thermal user plant 10 and an external circuit of a heat sink 20, which are only schematically shown. A first heat carrier fluid circulates in the external circuit of the thermal user plant 10, for example water or air, whereas a second heat carrier fluid circulates in the external circuit of the heat sink 20 which, also in this case, may be water or air.

As indicated above, the expression “heat sink” is understood to indicate a system capable of yielding or absorbing heat power without considerable variations of the average temperature thereof. Such system may be of the “open” type, such as for example a groundwater source, but it may also be of the “closed” type as in the case of a user heating/cooling plant. In both cases, anyway, there is provided an external circuit for the circulation of the heat carrier fluid resulting from the system defining the heat sink.

In situations of use of the heat pump unit 1 wherein both heat production and cold production are concurrently required, the heat sink 20 may therefore consist of a further thermal user plant capable of using the cooling or heating power otherwise disposed of through such heat sink. To this end, the thermal user plant may be a unit commonly called “four piped” or an air treatment unit comprising at least one heating battery and a post-heating battery.

For the purposes of the present invention, the expression “heat sink” indicates in general a system capable of absorbing thermal energy.

FIG. 1 shows a first preferred embodiment of the heat pump unit 1, in particular for heating, comprising a main circuit 2 for performing a main heat pump cycle HPCM with a respective operating fluid.

The main circuit 2 comprises: a main condenser S4 adapted to perform the condensation of the operating fluid at a higher pressure of the main heat pump cycle HPCM and intended to be connected to the external circuit of the thermal user plant 10 in a heating operating mode of the heat pump unit 1; a main evaporator S8 adapted to perform the evaporation of the operating fluid at a lower pressure of the main heat pump cycle HPCM and intended to be connected to the external circuit of the heat sink 20 in a heating operating mode of the heat pump unit 1; a compressor C2 adapted to bring the evaporated operating fluid from the lower pressure to the higher pressure of the main heat pump cycle HPCM, and expansion means L2—for example a lamination valve or other functionally equivalent known device—adapted to perform the expansion of the operating fluid from the higher pressure to the lower pressure of the main heat pump cycle HPCM.

The main circuit 2 further comprises a heat exchanger S3 connected downstream of the main condenser S4 and upstream of the expansion means L2, adapted to perform an undercooling of the operating fluid after the condensation of the same in the main condenser S4.

Within the scope of the present description and of the following claims, the expressions “upstream” and “downstream” are to be understood with reference to the directions of fluid circulation indicated in the figures by arrows and in general determined by the compressors, in the case of circuits for performing heat pump cycles, and by the circulation pumps in the case of the external circuits of the thermal user plants and of the heat sink, respectively.

The heat exchanger S3, moreover, is selectively connectable in a line FL1 arranged in the heat pump unit 1 for connecting the main condenser S4 to the external circuit of the thermal user plant 1 so as to be in series with the main condenser S4 in said external circuit.

This is preferably obtained by means of a three-way valve V10, preferably a modulating solenoid valve, arranged in line FL1 so as to selectively allow the partial or total by-pass of the heat exchanger S3. In particular, valve V10 is a “modulating” valve, i.e. such as to allow the dosing of the water intended to flow through the heat exchanger S3. That is, such valve V10 allows the flow rate intended for the heat exchanger S3 to be changed according to the requirements to allow, for example in heating spring and autumn, the heat carrier fluid circulating in the first thermal user plant 10 to receive more heat power even though the temperature of the same heat carrier fluid in output from exchanger S3 is always lower. In particular, as the winter season becomes colder, through the modulating valve V10 it is possible to decrease the percentage of the fluid FL1 flow rate intended to flow through S3. In this way, the overall temperature of the heat carrier fluid flowing through S3 and S4 will advantageously be higher even though the heat power carried by the same heat carrier fluid will be lower. On the contrary, as the winter season becomes less cold, through the modulating valve V10 it is possible to increase the percentage of the fluid FL1 flow rate intended to flow through S3. In this way, the overall flow rate flowing through S3 and S4 can advantageously receive more heat power even though the temperature of the same heat carrier fluid will be lower.

The heat exchanger S3 allows the use of the heat power resulting from an undercooling of the operating fluid of the main heat pump cycle HPCM for preheating the heat carrier fluid of the first thermal user plant 10 before it reaches the main condenser S4. As explained above, this leads to a significant improvement of the overall COP of the heat pump unit 1 in particular in operating conditions wherein a decrease in the temperature level of the thermal user plant 10 is acceptable, the heat power transferred thereto being equal, as may happen for example in a high temperature heating plant in spring and autumn.

Considering the main circuit 2, the heat exchanger S3 therefore is in series to the main condenser S4 and both are downstream of compressor C2. It is therefore clear that the heat exchanger S3 is an undercooler of the operating fluid in output from condenser S4. On the other hand, it is noted that the heat exchanger S3 and the main condenser S4 have both the function of heating the heat carrier fluid (preferably water) circulating in the external circuit of the thermal user plant 10.

According to the minimum temperature of the heat sink 20 available, the embodiment of the heat pump unit 1 shown in FIG. 1 may be used with low/mean or high temperature thermal user plants. For example, if a heat sink 20 is available with a mean temperature of no less than 7° C., as it happens for example in the case of groundwater or running water on the surface or in depth, seawater or lake water, waterworks water, wastewaters, etc., it is possible to serve thermal user plants that require temperatures up to 60-65° C., for example heating plants operating at low/mean temperature, such as plants with floor or ceiling radiating panels, fan coils, etc., or plants for the production of sanitary hot water.

On the other hand, if a heat sink 20 is available with a higher mean temperature (at least 30-35° C.), for example waste/cooling heat from industrial processes, hot spring, etc., it is possible to serve thermal user plants which require temperatures even higher than 60-65° C., for example heating plants operating at high temperature, such as plants with radiators, fan heaters, etc. which typically require delivery temperatures of 80° C. or higher, or plants for the production of sanitary hot water in all those situations where the hot water must be produced at temperatures considerably higher than 60° to prevent the possible occurrence of legionella (hospitals, swimming pools and sports centers, barracks, etc.).

The advantages resulting from the invention are more accentuated in the case of use of the heat pump unit 1 for serving thermal user plants operating at a high temperature since operating conditions may more frequently occur in such plants which allow a decrease of the temperature level whereto they operate at full load and which therefore, as already explained, allow the maximum advantage to be taken from the preheating that may be performed through the heat exchanger S3.

FIG. 2 shows a second preferred embodiment of the heat pump unit 1, which differs from that of FIG. 1 in the type of main circuit 2. In this case, the main circuit 2 comprises a first sub-circuit 2 a adapted to perform a higher temperature main heat pump cycle HPCM_HT with a respective operating fluid and a second sub-circuit 2 b adapted to perform a lower temperature main heat pump cycle HPCM_HT with a respective operating fluid. The first and second sub-circuits 2 a, 2 b are in cascading heat exchange relationship with each other to perform globally a two-stage main heat pump cycle HPCM.

In this case, the first sub-circuit 2 a comprises the main condenser S4, the heat exchanger S3, the expansion means L2 and compressor C2 described above with reference to the embodiment of FIG. 1, and an evaporator. The second sub-circuit 2 b comprises the main evaporator S8, already described above as well with reference to the embodiment of FIG. 1, a compressor C1, a condenser in heat exchange relationship with the evaporator of the first sub-circuit 2 b and the expansion means L1.

In the preferred embodiments shown herein, the condenser of the second sub-circuit 2 b and the evaporator of the first sub-circuit 2 a are integrated in a single heat exchanger device S7 for greater construction compactness and a better heat exchange efficiency. In any case, embodiments in which such components are separate and placed in thermal exchange relationship by an intermediate circuit for the circulation of a suitable heat carrier fluid are not excluded.

The embodiment of the heat pump unit 1 with two-stage main heat pump cycle HPCM finds an advantageous use in all those situations in which it is necessary to serve thermal user plants operating at a high temperature but having a low temperature heat sink available.

FIG. 3 shows a third preferred embodiment of the heat pump unit 1 which differs from that of FIG. 2 essentially by the provision, in the second sub-circuit 2 b for performing the lower temperature main heat pump cycle HPCM_LT, of a further heat exchanger S6 connected downstream of the heat exchanger device S7 and upstream of the expansion means L1.

The heat exchanger S6 is adapted to perform an undercooling of the operating fluid of the lower temperature main heat pump cycle HPCM_LT after the condensation thereof in the heat exchanger device S7, and is selectively connectable to the external circuit of the heat sink 20 so as to perform a preheating of the heat carrier fluid coming from the latter by means of the heat power released during said undercooling.

In particular, a first end of the heat exchanger S6 is connected to the second sub-circuit 2 b as described above and a second end of the heat exchanger S6 is connected upstream of the main evaporator S8 in a line FL2 arranged in the heat pump unit 1 for the connection to the external circuit of the heat sink 20. In such line FL2 there is also provided a valve V6, preferably a modulating solenoid valve, for adjusting the flow rate of heat carrier fluid of the heat sink 20 which crosses the heat exchanger S6, and thus the extent of the undercooling of the operating fluid in the higher temperature main heat pump cycle HPCM_LT. Line FL2 preferably also comprises a first manifold M1 connected upstream of the heat exchanger S6 and a second manifold M2 connected upstream of the main evaporator S8 and downstream of valve V6. Preferably, manifolds M1 and M2 are also connected by a bypass line BPL for bypassing the heat exchanger S6, provided with a valve V5, also preferably a modulating solenoid valve.

This further embodiment of the heat pump 1 therefore allows the heat power obtained with the undercooling of the operating cycle of the lower temperature main heat pump cycle HPCM_LT to be used for preheating the heat carrier fluid of the heat sink 20. This solution allows an advantageous increase in the COP as it advantageously increases the inlet temperature of the heat sink 20 heat carrier fluid to evaporator 8, i.e. decreasing the temperature difference between hot source and cold source.

Preferably, in order to keep a constant flow rate of the heat sink 20 heat carrier fluid in the main evaporator S8, a modulation or closure of valve V6 is compensated through a corresponding modulation or opening of valve V5.

Preferably, in this embodiment and in those described hereinafter, compressors C1 and C2 are variable flow rate compressors, for example cut-off step or inverter compressors. This ensures higher adaptability of the heat pump unit 1 to the possible unbalances in the thermal power exchange between higher temperature main heat pump cycle HPCM_HT and lower temperature main heat pump cycle HPCM_HT which may happen due to the undercooling. Such higher adaptability has a positive influence on the overall energy efficiency of the heat pump unit 1, all the other conditions being equal.

FIG. 4 shows a fourth preferred embodiment of the heat pump unit 1 which differs from that of FIG. 3 by the provision, in the first sub-circuit 2 a for performing the higher temperature main heat pump cycle HPCM_HT, of a further heat exchanger S5 connected downstream of the heat exchanger S3 and upstream of the expansion means L2.

Similar to heat exchanger S6, heat exchanger S5 is adapted to perform an undercooling of the operating fluid of the higher temperature main heat pump cycle HPCM_HT after the condensation thereof in the heat exchanger S4, and optionally after a first undercooling in heat exchanger S3, and is selectively connectable to the external circuit of the heat sink 20 so as to perform a preheating of the heat carrier fluid coming from the latter by means of the heat power released during said undercooling.

Preferably, heat exchanger S5 and heat exchanger S6 are arranged so as to perform the preheating of the heat carrier fluid of the heat sink 20 independently of one another, i.e. operating in parallel on two separate flows of such heat carrier fluid.

In particular, as shown in FIG. 4, heat exchanger S5 is connected in a first branch FL2′ of line FL2 for the connection to the external circuit of the heat sink 20. In such first branch FL2′ there is also provided a valve V7, preferably a modulating solenoid valve, for adjusting the heat carrier fluid flow rate (for example water) of the heat sink 20 which crosses the heat exchanger S5. The heat exchanger S6 and valve V6, on the other hand, are connected in a second branch FL2″, in parallel with the first branch FL2′, of line FL2. Exchanger S5 therefore exchanges heat between the heat carrier fluid of the heat sink (for example water) and the operating fluid (cooling fluid) circulating in the first sub-circuit 2 a and is used for preheating the inlet heat carrier fluid to evaporator S8 if the (cooling) operating fluid of the first sub-circuit 2 a still has residual heat not transferred through the main condenser S4 and the heat exchanger (undercooler S3) previously crossed by the same operating fluid. Actually, heat exchanger S5 is a further undercooler of the cooling fluid in output from heat exchanger S4.

Similar to what mentioned with reference to valve V6, also the modulation or closing of valve V7 may be compensated through a corresponding intervention on valve V5 in the bypass line BPL in order to keep a constant flow rate of the heat sink 20 heat carrier fluid in the main evaporator S8.

This embodiment of the heat pump 1 allows an undercooling of the operating fluid to be performed also in the higher temperature main heat pump cycle HPCM_HT after the condensation thereof and the use of the heat power thus released for preheating the heat sink 20 heat carrier fluid. Also this solution therefore allows an advantageous increase in the COP, increasing the inlet temperature of the heat sink 20 heat carrier fluid to evaporator 8, or decreasing the temperature difference between hot source and cold source.

FIG. 5 shows a fifth preferred embodiment of the heat pump unit 1 which differs from that of FIG. 4 in that the heat exchangers S5 and S6 are further selectively connectable to an external circuit of a further thermal user plant 12, in particular a thermal user plant operating at mean/low temperature, for example a heating plant with floor or ceiling radiating panels, a plant for the production of sanitary hot water, etc.

This is preferably obtained using a three-way valve V8, preferably a solenoid valve, and two valves V9 and V12, preferably modulating solenoid valves, arranged in such a way as to allow the connection of the second end of heat exchangers S5 and S6 alternately to the external circuit of the heat sink 20 or to the external circuit of the thermal user plant 12.

In particular, when the heat power released at the heat exchangers S5 and S6 must be used for serving the thermal user plant 12, the three-way valve V8 is diverted towards the external circuit of such plant, valves V6 and V7 are fully closed and valves V9 and V12 are fully or partly open. An adjustment of the opening degree of valves V9 and V12 allows the heat power transferred to the thermal user plant 12 to be adjusted.

On the contrary, when the heat power released at the heat exchangers S5 and S6 must be used for preheating the heat sink 20 heat carrier fluid, as in the embodiments described above with reference to FIGS. 3 and 4, the three-way valve V8 is diverted towards the external circuit of the heat sink 20, valves V9 and V12 are fully closed and valves V6 and V7 are fully or partly open.

In all the embodiments wherein the pairs of two-way valves V6+V9 and V7+V12 are provided, each of such pairs may be replaced by a three-way valve arranged so as to perform the functions described above of the corresponding two-way valves.

FIGS. 6A and 6B show a sixth preferred embodiment of the heat pump unit 1 adapted to operate for both heating and cooling, i.e. of the reversible type. To this end, in this embodiment there are provided switching means adapted to allow an exchange of connections of the external circuits of the thermal user plant 10 and of the heat sink 20 respectively with the main condenser S4 and with the main evaporator S8. Preferably, such switching means comprise two four-way valves V1 and V2, preferably solenoid valves, suitably arranged in the lines for the connection of the above external circuits to the main condenser S4 and the main evaporator S8.

In particular, in the operating configuration shown in FIG. 6A, corresponding to a heating operation (winter or autumn and spring), the external circuit of the thermal user plant 10 is connected to the main condenser S4 (and to the heat exchanger S3), whereas the external circuit of the heat sink 20 is connected to the main evaporator S8 in a manner totally similar to the embodiments described above. In the configuration shown in FIG. 6B, corresponding to a cooling operation (summer), the external circuit of the thermal user plant 10 is connected to the main evaporator S8 so as to provide such plant with the required cooling power whereas the external circuit of the heat sink 20 is connected to the main condenser S8.

It is noted that in the cooling operation, the use of heat exchangers S5 and S6 for the undercooling of the operating fluid respectively in the higher temperature main heat pump cycle HPCM_HT and in the lower temperature main heat pump cycle HPCM_LT allows a substantial increase of the useful cooling power without a corresponding increase of electrical power used, to the advantage of the overall energy efficiency. Numerical simulations carried out have shown that this embodiment of the heat pump unit 1 when operating for cooling allows EER values to be reached that are equal to 3.5-4.0, against a value of about 2.2 in the absence of undercooling.

The undercooling heat power released in this operating configuration at the heat exchangers S5 and S6 may advantageously be used for example for the production of sanitary hot water in a dedicated plant (schematized in FIG. 6B by the thermal user plant 12). If it is not possible to use the undercooling heat power, this shall be suitably disposed to the external environment.

Moreover, in the cooling operation, the use of heat exchanger S3 is not generally needed and the three-way valve V10 is therefore diverted so as to exclude such heat exchanger from line FL1.

FIGS. 7A and 7B show a seventh preferred embodiment of the heat pump unit 1 which differs from that of FIGS. 6A and 6B in that it can also serve, in a dedicated manner, both in a heating operating configuration (FIG. 7A), and in a cooling operating configuration (FIG. 7B), a thermal user plant 13 for the production of sanitary hot water, in addition to the thermal user plants 10 and 12. In particular, this embodiment allows a thermal user plant to be served for the production of high temperature (higher than 60° C. for preventing the possible occurrence of legionella) sanitary hot water.

The embodiment shown in FIGS. 7A and 7B, by way of an example, envisions that the heat exchange with the thermal user plant 13 indirectly takes place at a heat accumulator (boiler) 13 a, but other solutions known by the man skilled in the art are also possible to connect the heat pump unit 1 to the external circuit of such thermal user plant.

Compared to the embodiment of FIGS. 6A and 6B, connections are further provided in this case for an external circuit of the thermal user plant 13 and two three-way valves V3 and V11, preferably solenoid valves.

The three-way valve V3 is arranged so as to allow, in the heating operating configuration (FIG. 7A), the connection of line FL1 alternately to the external circuit of the thermal user plant 10 or to the external circuit of the thermal user plant 13. In this way it is possible to use the heat power released at the main condenser S4 alternately for heating or production of high temperature sanitary hot water.

The three-way valve V11 is arranged so as to allow, in the cooling operating configuration (FIG. 7B), the connection of line FL1 alternately to the external circuit of the thermal user plant 13. In this way it is possible to use the heat power released by the main condenser S4 for producing high temperature sanitary hot water rather than dispersing such power at the heat sink 20.

FIGS. 8A and 8B show an eighth preferred embodiment of the heat pump unit 1 which, compared to the embodiment of FIGS. 7A and 7B, in addition allows also low temperature sanitary hot water requirements to be met through the thermal user plant 13 for the production of sanitary hot water already mentioned. The embodiment of FIGS. 8A and 8B differs from that of FIGS. 7A and 7B in particular by the presence of a further three-way valve V4, preferably a solenoid valve.

The three-way valve V4 is arranged so as to allow the selective connection of lines FL2′ and FL2″, wherein the heat exchangers S5 and S6 are connected, also to the external circuit of the thermal user plant 13 for the production of sanitary hot water, so as to create a closed circuit therewith. In this way it is possible to use the heat power released at the two heat exchangers S5 and S6 alternately for producing sanitary hot water both in the heating operating configuration (FIG. 8A) and in the cooling operating configuration (FIG. 8B), or for preheating the heat sink 20 heat carrier fluid in the heating operating configuration.

For an optimum operation of this embodiment in the cooling operating configuration (FIG. 8B) it is suitable to provide, externally to the heat pump unit 10, means for bypassing the external circuit of the thermal user plant 10, intended for cooling in this operating configuration. Such means preferably comprise a three-way valve V13, preferably a solenoid valve, arranged between the external circuit of the thermal user plant 10 and the external circuit of the thermal user plant 13. The external three-way valve V13, together with the already described three-way valve V11 of the heat pump unit 1, allows line FL1 to be connected to the external circuit of the thermal user plant 13 bypassing the external circuit of the thermal user plant 10.

In particular, in the cooling operating configuration shown in FIG. 8B, the three-way valve V11 connects line FL1 to the external circuit of the thermal user plant 13 whereas the external there-way valve V13 allows the external circuit of the thermal user plant 10 to be bypassed. In this way it is possible to use the heat power released by the main condenser S4 for producing high temperature sanitary hot water rather than dispersing such power at the heat sink 20. This operating mode requires circuit FL1 to be a closed circuit.

FIGS. 9A and 9B show a ninth preferred embodiment of the heat pump unit 1, which differs from that of FIGS. 8A and 8B mainly in that it comprises a further heat exchanger connected in sub-circuit 2 a and adapted to perform an undercooling of the operating fluid of said higher temperature main heat pump cycle HPCM_HT as well as a secondary circuit 3 adapted to perform a secondary heat pump cycle HPCS with a respective operating fluid and in heat exchange relationship with sub-circuit 2 a at said further heat exchanger.

The further heat exchanger is connected in sub-circuit 2 a downstream of the main condenser S4 and preferably, of the heat exchanger S3, and upstream of the heat exchanger S5.

The secondary circuit 3 for performing a secondary heat pump cycle HPCS comprises a secondary condenser S1 adapted to perform the condensation of the operating fluid at a higher pressure of the secondary heat pump cycle HPCS and intended to be connected to the external circuit of the thermal user plant 10 or to the external circuit of a further thermal user plant, separate from the latter and from the thermal user plants 12 and 13 mentioned above; a secondary evaporator adapted to perform at least the evaporation of the operating fluid at a lower pressure of the secondary heat pump cycle HPCS and in heat exchange relationship with said further heat exchanger of sub-circuit 2 a for transferring heat power released by the operating fluid of the main heat pump cycle HPCM during said undercooling to the operating fluid of the secondary heat pump cycle HPCS; a compressor C3 adapted to bring the evaporated operating fluid from the lower pressure to the higher pressure of the secondary heat pump cycle HPCS, and expansion means L3—for example a lamination valve or another functionally equivalent known device—adapted to allow the expansion of the operating fluid from the higher pressure to the lower pressure of the secondary heat pump cycle HPCS.

In the embodiment of FIGS. 9 a and 9 b, the further heat exchanger of sub-circuit 2 a and the secondary evaporator of the secondary circuit 3 are integrated in a single heat exchanger device S2 for greater construction compactness and a better heat exchange efficiency. In any case, embodiments wherein such components are separate and placed in thermal exchange relationship by an intermediate circuit for the circulation of a suitable heat carrier fluid are not excluded.

Compressor C3 preferably is a variable flow rate compressor, for example a cut-off or step or inverter compressor. This allows the extent of the operating fluid undercooling in the main heat pump cycle HPCM at the heat exchanger device S2 and accordingly, the heat power that can be provided at the secondary condenser S1 to be controlled without stressing the compressor with an excessive repetition of on-off cycles.

By means of the secondary circuit 3 and the relevant secondary heat pump cycle HPCS described above it is possible to raise the temperature by at least a fraction of the heat power released upon the undercooling of the operating fluid in the main heat pump cycle HPCM, bringing it to values equal or close to those of the heat power released at the main condenser S4. In this way, the above fraction of undercooling heat power becomes usable by the thermal user plant 10 or by another and separate thermal user plant operating at medium or high temperature also in operating conditions wherein such thermal user plants must operate at the expected maximum temperatures. Thanks to the fact that such result may be obtained minimizing the additional energy consumption related to compressor C3, as already explained above, the increase in the overall useful heat power leads to an increase in the overall COP of the heat pump unit 1.

When both the main condenser S4 and the secondary condenser S1 are intended to serve the same thermal user plant, as in the embodiment of FIGS. 9A and 9B, they are reciprocally connected in series, with the secondary condenser S1 upstream, in line FL1 for the connection to the external circuit of such thermal user plant. In this way it is possible to use the secondary condenser S1 for performing a preheating of the heat carrier fluid of the thermal user plant, and the main condenser S4 for completing the heating up to reaching the required delivery temperature.

As already explained above, since in this case the secondary condenser S1 must only contribute to a part of the heating, the heat power to transfer to the thermal user plant being equal, it is possible to decrease the condensation temperature in the secondary heat pump cycle HPCS, obtaining a simultaneous decrease of the compression work required in such cycle and thus, a further improvement of the overall COP of the heat pump unit 1.

Therefore, with reference to the heating operating configuration (FIG. 9A) of the ninth embodiment described above, in operating conditions wherein the temperature level in the thermal user plant 10 must be maximum (for example in full winter), the three-way valve V10 is preferably diverted so as to connect the secondary condenser S1 in line FL1 and exclude the heat exchanger S3. Advantageously, the heat power provided by the undercooling of the operating fluid of the higher temperature main heat pump cycle HPCM_HT can thus be transferred to the thermal user plant 10 at a higher temperature, thanks to the secondary heat pump cycle HPCS performed in the secondary circuit 3 (active compressor C3).

In operating conditions wherein the temperature level in the thermal user plant 10 may be reduced, the three-way valve V10 is on the contrary preferably diverted so as to connect the heat exchanger S3 in line FL1 and exclude the secondary condenser S1, and the secondary circuit 3 is deactivated (compressor S3 off). Advantageously, in this case, the heat power provided by the undercooling of the operating fluid of the higher temperature main heat pump cycle HPCM_HT is transferred to the thermal user plant 10 directly, without heat increase, through the heat exchanger S3. The adjustment of the delivery temperature for the thermal user plant 10 takes place through the modulation of the three-way valve V10. A further advantage may be obtained by shutting or reducing the number of revolutions of compressors C1 and C2 in order to reduce the heat power delivered.

In the heating operating configuration of the ninth embodiment, shown in FIG. 9A, the use level of the heat exchanger S5, i.e. the fraction of undercooling heat power used therein with respect to the total available, depends on the corresponding use level of the heat exchanger device S2 and of heat exchanger S3.

In particular, the use level of heat exchanger S5 is maximum when the three-way valve V10 is diverted so as to exclude the heat exchanger S3 and the secondary circuit 3 is not active (compressor C3 off). On the contrary, the use level is null when all the undercooling heat power is used in the heat exchanger device S2 (for example in full winter operating conditions) or in heat exchanger S3 (for example in autumn and spring operating conditions). In this case, the heat exchanger S5 is excluded from line FL1 by closing valve V7. In intermediate situations, the use level of the heat exchanger S5 is partial and valve V7 must modulate accordingly. In the cooling operating configuration of the ninth embodiment, shown in FIG. 9B, the secondary circuit 3 for performing the secondary heat pump cycle HPCS typically is not active (compressor C3 off). Alternatively, for example in use situations wherein the production of large amounts of sanitary hot water is required even in hot seasons, it is possible to provide also for the transfer of the power released at the secondary condenser S1 to a plant for the production of sanitary hot water.

In all the embodiments described, the heat pump unit 1 preferably comprises also a programmable control unit not shown in the figures. In particular, such control unit may be suitably programmed for controlling the opening/closing, the modulation or diverting of the valves as well as the switching on/off, the shutting degree or the number of revolutions of the compressors present in each embodiment of the heat pump unit 1.

The operating fluids used in the various heat pump cycles performed in the heat pump unit 1 may be equal to or different from, each other.

Operating fluids are preferably selected that allow the following advantageous features to be combined for the operation of the heat pump unit 1:

-   -   limit curves, and in particular lower limit curve, in diagrams         h-p highly inclined in the direction of the increasing         enthalpies;     -   high specific heat of the operating fluid at the liquid state         with respect to the latent condensation/evaporation heat;     -   high specific heat of the operating fluid at the vapor state         with respect to the latent condensation/evaporation heat.

The first two features mentioned above are particularly important for embodiments or operating conditions that use strong undercooling whereas the third one is particularly important for all the embodiments or operating conditions that use strong overheating.

In particular, in order to obtain the best performance of the heat pump unit 1, the following operating fluids have proven to be particularly advantageous: (E)-2-butene, (Z)-2-butene, 1-butylene, dimethyl ketone, methylacetylene, methyl alcohol, methylpentane, methylpropene, n-hexane, R1270, R290, R600, R600a, R601, R601a, RE-170, tetramethylmethane or RC-270.

Besides having at least one or more of the desired features listed above, these operating fluids have the advantage of being so-called “natural” cooling fluids, i.e. not harmful for the environment from the viewpoint of negative effects on the stratospheric ozone, or from the viewpoint of the greenhouse effect.

If the type of operating fluid selected, in particular for its hydrocarbon nature, poses safety issues (fire hazard) in the cases in which the heat pump unit 1 must be installed in underground or basement rooms, the latter is preferably provided also with means for the detection and evacuation of gas leaks.

FIG. 10 shows an embodiment of the heat pump unit 1 comprising a system for the detection and evacuation of gas leaks. By way of an example, the configuration of the heat pump unit 1 shown corresponds to the second embodiment described above with reference to FIG. 2.

The system for the detection and evacuation of gas leaks comprises at least one gas detector 31, positioned as close as possible to the bottom of the heat pump unit 1 and ventilation means 32, activatable by the gas detector 31 and arranged so that the suction thereof is also close to the bottom of the heat pump unit 1, whereas the delivery thereof is connected to a gas evacuation conduit in communication with the external environment. Optionally, there may be provided a dedicated control device 34 adapted to receive signals from the gas detector 31 and to control the ventilation means 32 accordingly. The control device 34 may also control sound and/or light warning means 35, if provided, and/or be configured for sending alarm signals to an optional external monitoring/supervision system (not shown). The functions of the control device 34 may also be carried out by the programmable control unit of the heat pump unit 1.

Of course, a man skilled in the art may use the technical features of the heat pump unit 1 of the invention disclosed with reference to the preferred embodiments described above also in different combinations in order to meet specific and contingent application requirements. 

1. Heat pump unit comprising at least one main circuit adapted to perform a main heat pump cycle with a respective operating fluid, said at least one main circuit comprising: a main condenser adapted to perform the condensation of the operating fluid of said main heat pump cycle and intended to be connected to an external circuit of a first thermal user plant in a heating operating mode of said heat pump unit; a first heat exchanger, connected downstream of said main condenser and upstream of expansion means of said at least one main circuit, adapted to perform an undercooling of the operating fluid of said main heat pump cycle after the condensation of the same in said main condenser, and a main evaporator adapted to perform the evaporation of the operating fluid of said main heat pump cycle and intended to be connected to an external circuit of a heat sink in a heating operating mode of said heat pump unit, characterized in that said first heat exchanger is selectively connectable to the external circuit of said first thermal user plant so as to be in series with said main condenser in said external circuit.
 2. Heat pump unit according to claim 1, wherein said main circuit comprises a first sub-circuit adapted to perform a higher temperature main heat pump cycle with a respective operating fluid and a second sub-circuit adapted to perform a lower temperature main heat pump cycle with a respective operating fluid, wherein said first and second sub-circuits are in cascading heat exchange relationship with each other so as to perform globally a two-stage main heat pump cycle, and wherein said main condenser and said first heat exchanger are connected in said first sub-circuit and said main evaporator is connected in said second sub-circuit.
 3. Heat pump unit according to claim 2, wherein said second sub-circuit comprises a second heat exchanger connected downstream of a condenser and upstream of expansion means of said second sub-circuit, said second heat exchanger being adapted to perform an undercooling of the operating fluid of said lower temperature main heat pump cycle after the condensation thereof and being selectively connectable to the external circuit of said heat sink so as to perform a preheating of a heat carrier fluid coming from said heat sink by means of heat power released during said undercooling by the operating fluid of said lower temperature main heat pump cycle.
 4. Heat pump unit according to claim 3, wherein said second heat exchanger is further selectively connected to an external circuit of a second thermal user plant.
 5. Heat pump unit according to claim 3, wherein said first sub-circuit comprises a third heat exchanger connected downstream of said first heat exchanger and upstream of expansion means of said first sub-circuit, said third heat exchanger being adapted to perform an undercooling of the operating fluid of said higher temperature main heat pump cycle after the condensation thereof and being selectively connectable to the external circuit of said heat sink so as to perform, preferably independently from said second heat exchanger, a preheating of a heat carrier fluid coming from said heat sink by means of heat power released during said undercooling by the operating fluid of said upper temperature main heat pump cycle.
 6. Heat pump unit according to claim 5, wherein said third heat exchanger is further selectively connectable to the external circuit of said second thermal user plant.
 7. Heat pump unit according to claim 2, comprising a fourth heat exchanger connected downstream of said main condenser and upstream of said third heat exchanger, said fourth heat exchanger being adapted to perform an undercooling of the operating fluid of said higher temperature main heat pump cycle after the condensation of the same, and a secondary circuit adapted to perform a secondary heat pump cycle with a respective operating fluid, said secondary circuit comprising: a secondary evaporator adapted to perform at least the evaporation of the operating fluid of said secondary heat pump cycle and in heat exchange relationship with said fourth heat exchanger to transfer heat power released by the operating fluid of said main heat pump cycle during said undercooling to the operating fluid of said secondary heat pump cycle; a secondary condenser adapted to perform the condensation of the operating fluid of said secondary heat pump cycle and intended to be connected to the external circuit of said first thermal user plant or to an external circuit of a third thermal user plant, different from said first and second thermal user plants.
 8. Heat pump unit according to claim 1, comprising switching means, adapted to allow an exchange of connections of the external circuits of at least said first thermal user plant and said heat sink respectively with at least said main condenser and with said main evaporator
 9. Heat pump unit according claim 1, wherein the operating fluid of said main heat pump cycle, or the operating fluids of said higher temperature main heat pump cycle and of said lower temperature main heat pump cycle respectively, and the operating fluid of said secondary heat pump cycle are selected from the group consisting of: (E)-2-butene, (Z)-2-butene, 1-butylene, dimethyl ketone, methylacetylene, methyl alcohol, methylpentane, methylpropene, n-hexane, R1270, R290, R600, R600a, R601, R601a, RE-170, tetramethylmethane or RC-270.
 10. Heat pump unit according to claim 1, comprising means for detecting and evacuating gas leaks.
 11. System for heating/cooling environments and/or for producing sanitary hot water comprising a heat pump unit according to claim
 1. 12. Heat pump unit according to claim 4, wherein said first sub-circuit comprises a third heat exchanger connected downstream of said first heat exchanger and upstream of expansion means of said first sub-circuit, said third heat exchanger being adapted to perform an undercooling of the operating fluid of said higher temperature main heat pump cycle after the condensation thereof and being selectively connectable to the external circuit of said heat sink so as to perform, preferably independently from said second heat exchanger, a preheating of a heat carrier fluid coming from said heat sink by means of heat power released during said undercooling by the operating fluid of said upper temperature main heat pump cycle.
 13. Heat pump unit according to claim 12, wherein said third heat exchanger is further selectively connectable to the external circuit of said second thermal user plant. 